The Czochralski (CZ) process is probably the most widely used technique for growing crystal ingots used in the manufacture of IC chips. In the CZ process, a "crystal puller" system grows solid, crystal ingots from a melted form of charge material. High-quality ingots are substantially free of defects, have a uniformity of characteristics throughout the ingot, and are largely uniform from one ingot to the next.
A modern crystal puller typically includes a growth chamber, containing a crucible, heated by a furnace. The crucible holds a charge material, such as silicon, and the furnace heats the charge to a melted state (the "melt"). A crystal lifting mechanism holds a "seed" at the end of the rod or cable and lowers the seed to contact the melt. Upon contacting the melt, the seed causes a local decrease in melt temperature, which causes a portion of the melt to crystallize around the seed. The seed is then slowly raised out of the melt by the lifting mechanism. As the seed is withdrawn, the portion of the newly-formed crystal that remains within the melt essentially acts as an extension of the seed and causes more melt to crystallize around the seed and crystal. The lifting is continued until the ingot is grown to the desired size.
To grow high-quality, defect-free ingots, modern crystal pullers are essentially hermetic. Any contaminants introduced into the melt can cause crystal defects. Thus, for example, special seals, known in the art, are used prevent contaminants from entering the growth chamber through various conduits into the chamber, such as the conduit for the lifting cable.
Moreover, to grow high-quality, defect-free ingots, modern crystal pullers monitor and control various growth parameters. For example, a control system controls the rate at which the lifting mechanism raises the crystal, and the rate at which the crystal and crucible are rotated.
One of the growth parameters of particular importance are the thermogradients at the melt/crystal interface. Changes in the thermogradients at the melt/crystal interface are known to affect crystal quality. Thus, it is believed to be desirable to keep the thermogradients uniform during the growing process.
Unfortunately, controlling the thermogradients has proven to be difficult, as the crystal growing process itself affects the thermogradients. In short, the crystal growing process effectively removes melt from the crucible and adds it to an ingot. Without more, this transfer of material from the melt to the ingot will lower the melt-level and, in turn, change the thermogradients at the melt/crystal interface.
To control the melt-level, the art has used two general approaches. One approach raises the crucible so that the melt surface in the crucible maintains a substantially absolute level, even though the melt-level will vary relative to the crucible. Another approach adds charge material to the crucible so that the melt-level substantially maintains an absolute level and also maintains the same relative level within the crucible.
To be effective, each of the above approaches requires knowledge of the melt-level so that the appropriate control may be made. That is, both of the above general approaches require knowledge of the melt level before either the crucible is raised or charge material is added.
To determine the melt-level, several systems have been proposed. For example, one type of system assumes that the melt volume will decrease at a rate related to the known rate at which the seed is raised by the lifting mechanism. Another proposal uses an electrical circuit that includes a platinum rod to directly contact the melt surface; the characteristics of the circuit depend upon the amount of platinum rod used. Each of these systems has its unique disadvantages, such as inaccurate estimations or risk of contamination.
Another technique uses a laser and light detection system to directly monitor the melt-level. The laser technique provides relatively accurate estimates of the melt-level and essentially no risk of introducing contaminants to the melt.
More particularly, referring to FIG. 1A, with the laser technique, a light source 1 emits a light beam toward the melt 2. The light beam reflects upward off the melt toward a light detection system 3. As can be readily seen, because the melt-level partially defines the geometry of the system, the position at which the light beam strikes the detection system 3 depends on the melt-level.
Unfortunately, actual melt systems are not as simple as the flat surface model of FIG. 1A. Rather than having a flat surface, actual melt systems have surfaces that undulate, as suggested in FIG. 1B. The undulations are a consequence of the dynamics of the system and of inherent properties of the system and materials. Typically those undulation have a frequency in the range of about 10-100 HZ and a height in the range of about 1-10 inches. Moreover, it is now believed that at least some of these undulations cannot be feasibly prevented. Consequently, a truly useful system must account for the undulations when determining the melt-level.
FIG. 1B illustrates a somewhat more accurate model of the melt system, including an undulating melt surface 5. As is seen in FIG. 1B, the angle .alpha. (measured from the normal z) at which the light beam reflects off the melt surface depends upon the melt-level and also upon what portion of a surface wave the beam contacts. Because the undulating surface is a time-varying signal having a time-varying component .DELTA..alpha., the angle of reflection is also a time-varying signal, having a time-varying component 2.DELTA..alpha.. Referring to FIG. 1C, it can be seen that the angle of reflection not only varies along the Y, but also along the X direction. Consequently, over a given time period, the reflected light beam moves about within a given area, called a target area, shown as area 25 in FIG. 2. The pattern 20 represents the time-elapsed trajectory of a hypothetical reflected light beam and is essentially a random pattern that falls within target area 25.
The trajectory 20 is sampled by the detector 3, which provides information indicating where the light beam strikes the detector. A control system (not shown) analyzes the information to determine an average position of the reflected beam and to thereby estimate the melt-level. The average position, in effect, is a "DC"-like component of the melt and the ripples are an "AC"-like component.
Typical prior art light detection systems 3 use discrete sensor elements in two basic configurations: FIG. 3A, for example, illustrates a conventional two-dimensional (2-D) arrangement 30 of sensor elements 31; and FIG. 3B, on the other hand, illustrates a conventional one-dimensional (1-D) arrangement 35. Although these light detection systems have proven useful, certain shortcomings have recently been appreciated. For example, because the target area 25 can be somewhat large, the light detector array 3 must also be large to avoid the risk of missing samples of information.
A large size array 30 requires more sensor elements to "capture" the reflected light. The larger number of elements increases the cost of the system and also reduces reliability, as there are more elements that can fail.
Alternatively, an undersized array 35 (see FIG. 5) may be used but, as readily seen increases the risk of missing samples of information. This will reduce the confidence in the information provided by the detections system. In order to overcome this drawback another prior art system disclosed in U.S. Pat. No. 5,286,461, uses a linear detector 35 which does not cover the entire target area. Light from source 400 is reflected off the melt surface and passes through an aperture 40 in a barrier 415. The light that passes through the aperture 40 also passes through a band-pass filter to eliminate the light emitted by the hot melt surface. The remaining light which is indicative of the reflected light beam, impinges on the detector 35.
Linear detector 35 indicates not only when a light beam is incident on it, but also indicates where the beam strikes the detector. Thus, as shown in FIG. 5, which indicates the sensor 35 located in the target area 25, the sensor will only produce and output when the beam is incident on the sensor and the output will be indicative of the vertical position on the sensor 35. A computer system (not shown) is utilized to implement an algorithm that processes the output signals from the detector which are continually sampled over time. The algorithm disregards the location data sent from the detector if it determines that the light beam was not incident on the detector for a given sample. By analyzing location data only for samples generated when the light beam was incident on the detector the algorithm determines an average location of the melt level for a predetermined number of samples and assumes that this average is representative of the location of the actual melt level. This system improves over the previous prior art systems, but the linear sensor is expensive and the system requires a significant time and computing power to average the samples and produce a result.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for determining the melt-level within a crucible utilized in a crystal growing system.